Recently the Environmental Protection Agency (EPA) announced that diesel exhaust fluid (DEF) quality sensors are not required on trucks. The American Trucking Associations (ATA) and others have argued that this sensor is not needed and is a major cost to the industry.
When the sensor fails, it adversely affects fleets in that it creates downtime and increases maintenance and repair costs. Fleets also have reported false readings of the diagnostics. Once the electronic control system believes there is a failure, the engine power is derated, and in some cases, this leaves trucks on the side of the road.
According to EPA, DEF-related failures have been costing the industry billions of dollars.
While the removal will save money, it is worth asking whether this is a step backwards for the environment. Are we trading off dollars for dangerous nitrogen oxides (NOx) emissions?
Let’s look at the purpose of the sensor and then determine if other alternate forms of monitoring and sensing can ensure emissions compliance.
What is the purpose of the sensor?
Selective catalyst reduction (SCR) systems were introduced in 2010 in the United States to meet the NOx requirements. With an SCR system, DEF is injected into the exhaust stream, and this enables the SCR catalyst to convert harmful NOx into nitrogen and water vapor.
If it works properly, the SCR system is very effective. However, many components can fail. In addition to failures, the SCR system will not function properly with poor quality DEF. Poor quality DEF can mean the DEF does not meet proper specifications. Therefore the storage and handling of DEF is critical. It should be stored in containers and environments where the temperatures are between 32°F to 77°F. DEF freezes at temperatures lower than 12°F, hence the need for a DEF tank heater; at temperatures greater than 86°F, it can degrade. Its shelf life is two years, assuming it is being stored in proper containers and with proper sealing mechanisms to prevent water ingestion. If these standards are not followed, the DEF will degrade. Finally, there is always concern about tampering, such as the tank being filled with water.
To ensure high quality DEF is present in the system, the sensor became mandatory back in the 2010 to 2013 timeframe.
Now that the sensor is not required, will trucks emit more NOx?
The goal of the emission control system is to ensure that exhaust leaving the tailpipe is clean, meeting all environmental regulations. It is not to determine if there is poor quality DEF in the tank.
Figure 1 shows an SCR system with the DEF doser positioned just before the SCR catalysts on the right-hand side of the diagram. As the exhaust flow moves from the engine to the SCR system (left to right on the diagram), DEF is injected. The DEF enables the SCR catalyst to reduce the NOx.
The diagram also shows two NOx sensors — one at the inlet and one at the outlet. By reading these sensors, the control system can detect SCR ineffectiveness and other issues. A DEF quality sensor is not needed to ensure emissions compliance. The only exception is that a DEF quality sensor will likely detect an issue earlier than what the NOx sensors can detect. This lag in detection is probably on the order of an hour or two.
It is also worth noting that the NOx sensing strategy is not just a few lines of code. On Board Diagnostics (OBD) requirements result in an extremely thorough evaluation as to whether the system is working properly, and the NOx sensing approach is a part of this.
Figure 1: illustration of SCR system with Nox sensor at inlet and outlet (Source)
A little more about OBD
EPA and California Air Resource Board (CARB) require OBD. The OBD system is highly complex and thorough. There are numerous OBD monitors that are required. Some examples of the purpose of these monitors are listed here.
To measure the efficiency of NOx conversion, comparing inlet and outlet NOx. If the outlet NOx sensor is not seeing a significant reduction from the inlet NOx sensor, fault codes and derates are induced.
To measure the output NOx sensor and compare it to the expected level that is in the control module; the expected value is based on numerous parameters and tables in the control module. DEF dosage rate, temperatures, pressures, and other operating conditions are all monitored. If there is a significant difference in the measured NOx and expected NOx, fault codes and derates are induced.
To check the integrity of the NOx sensor (referred to as NOx rationality check.) The NOx sensor itself is evaluated. If there is a failure, faults and engine derates are induced.
This is not a complete list of OBD monitors, but it’s intended to illustrate the thoroughness of OBD.
Engine manufacturers need to prove to the regulators that the OBD system is working properly. They must provide test data as well documentation. Below are some examples of the test data.
- Malfunction testing: After conducting a Failure Modes Effect Analysis (FMEA) which assesses all potential failures, the manufacturer must create the failures and prove that the OBD system catches them. Some examples would be to run with a catalyst that is partially plugged, a catalyst which has degraded, or poor DEF quality. The manufacturer must demonstrate that OBD works when these failures are introduced.
- In-use tests: Manufacturers must track how often the monitor runs in the real world and show that the operation is realistic relative to driving conditions.
- In-use performance: Testing must be conducted on new vehicles and vehicles which have operated in real-world conditions. Vehicles which have run in real-world conditions, typically are field test units which have run for a year or more hauling real freight. Also, degradation testing is performed in test cells where the engine runs for thousands of hours, and this data is used to quantify the potential degradation of the SCR system over time.
Documentation and certification also are quite involved. Some examples of what is required include description of the monitoring strategy, criteria used to set thresholds, and a list of any deficiencies in the OBD effectiveness. All of this needs to be provided to the regulators.
I checked with a few colleagues who informed me that OBD requires tens of thousands of lines of code, at a minimum. One person indicated that hundreds of thousands of lines of code were needed. As for the documentation, they indicated that 5000 to 10,000 pages often are needed to properly develop and certify the OBD system. These numbers might not be perfectly accurate, but they certainly put the complexity and thoroughness of OBD in context.
Complexity
When we first launched electronic fuel injected systems, we said with pride that these systems were more complex than the Apollo spacecraft. This was back in the 1980s.
The complexity of engines, aftertreatment, and controls has grown astronomically over the years. One quick search on Chat GPT indicated that there are 100 million lines of code in modern day vehicles. While none of these are exact numbers, my intuition says that 60% to 70% of this is related to the powertrain controls and diagnostics. (Just wait till we see autonomous vehicles!)
A principle that I learned early in my career is that the simpler the design, the better. Simple designs have fewer components that can meet the same set of requirements. They typically are more reliable than more complex solutions. We normally think of reliability in terms of the lack of parts failing, but this also applies to emissions compliance. The simpler the design — assuming it has been designed and validated properly — the less likely the chances of emissions noncompliance.
Life is full of making trade-offs. This also is true in the design of products. Engineers are challenged to design products that are reliable, low cost, and environmentally friendly. Our industry has come a long way in reducing NOx, PM, and CO2. We do not want to go backwards. This change in policy appears to be a fair trade-off by eliminating the need for the DEF quality sensor while still being assured there will be no significant emissions compliance issues.
